The reproductive effects of melatonin in the Syrian hamster may be due to its action upon steroid receptors in peripheral, pituitary or brain tissues. By examining the effect of melatonin in a peripheral target tissue, the possible mechanisms for melatonin action may be less complex than that seen centrally. We have previously observed that melatonin caused a transient increase in estrogen receptors in uteri from juvenile hamsters. We now find that adult ovariectomized hamsters have decreased concentration of estrogen receptors following melatonin treatment. Further, 3H-estradiol uptake was reduced in the uteri of melatonin treated adults and finally, melatonin partially inhibited estrogen-stimulated uterine growth. These observations suggest that melatonin affects estrogen action and this effect is dependent on the sexual development of the animal.